The generally high line-to-line correlation of conventional television video signals makes possible the separation of chrominance and luminance components of composite video signals with interline comb filters. The cancellation of luminance in comb filtered chrominance signals and the cancellation of chrominance in comb filtered luminance signals can only be achieved when this underlying assumption holds true. When it does not, both the comb filtered luminance and the comb filtered chrominance signals are contaminated with cross components.
A particularly objectionable artifact is present in reproduced images when line-to-line chrominance changes occur. This artifact is manifested as a dot pattern (known as hanging dots) which appears along horizontal edges of displayed images. The dots have a spatial frequency corresponding to the subcarrier temporal frequency. The dot pattern results from incomplete cancellation of the chrominance component from the comb filtered luminance component. The contaminating chrominance signal is generally a narrow band signal having a frequency equal to the color subcarrier frequency.
Arthur Kaiser in the article "Comb Filter Improvement with Spurious Chroma Detection", SMPTE Journal, Vol. 86, No. 1, 1977, pp. 1-5, describes apparatus for reducing the effects of uncancelled chrominance in the luminance component. This apparatus compares composite video signals separated by two lines to detect vertical image changes. When changes occur, the comb filtered luminance signal is selectively low-pass filtered to attenuate the band of frequencies occupied by the chrominance signal, thereby substantially eliminating the dot pattern. However, the low-pass filter also eliminates all of the high frequency luminance components.